There was a vendor at the Southeastern Regional Bonsai Show that had some stones for sale. I imagine you would find the same to be true at most large shows. Check out the next St Louis show or the Chicago show in August.

It depends on what you're looking for. Kemin Hu's website is wonderful (spirit-stones, as referred to by Harry), but they are not traditional suiseki. She is mostly selling what would be classified as gongshi (scholar's rocks). She does have wonderful stones, and there's usually good deals during the holiday season.

If you're looking for Japanese style stones (suiseki), then Sean Smith would be a great resource.

Finally, you could by from Chiara in Italy. Although I have no personal experience with purchasing from her, I do know some that have, and they have reported positive experiences (in fact, it was a friend of hers in Virginia, he's a well regarded stone enthusiast and I trust his judgment). Here's her website: http://www.padrini.it/zen/shop-en.htm. The stones are mostly (guessing a little) ligurian limestone.

Sean is one of the most knowledgable Americans on viewing stones in general and suiseki in particular. From $20,000.00 Japanese stones to $40 American stones in the suiseki tradition, Sean is the man to talk to. He also sells accessories for stone display as well as making stands for suiseki.

DISCLAIMER. I'm a good friend of Sean's and collect with him whenever I can. I don't make any money from promoting him; in fact, I've spent more money than I probably should.